ES251660A1 - Preparation of phthalic acids by oxidation - Google Patents

Abstract

<PICT:0864106/IV (b)/1> An oxidation process for the production of phthalic acids and esters comprises continuously introducing a xylene and a lower alkyl toluate into the first of a series of oxidation zones, continuously oxidising said xylene and said alkyl toluate in said zone in the liquid phase by passing an oxygen-containing gas p therethrough, continuously withdrawing oxidate containing unreacted alkyl toluate from said first zone, passing said withdrawn oxidate to a second oxidation zone while simultaneously introducing additional xylene to said second zone, continuing the oxidation of said withdrawn oxidate in the presence of said additional xylene in said second oxidation zone in the liquid phase by passing an oxygen-containing gas therethrough and continuously withdrawing oxidate containing a monoalkyl phthalate and a phthalic acid from said second oxidation zone, the amount of xylene introduced into each oxidation zone being proportioned to provide approximately equal rates of oxidation in each zone, in terms of acid number increase per hour, and the total amount of xylene introduced being in excess of that stoichiometrically equivalent to the amount of lower alkyl toluate introduced in the process. The terms "phthalic acid," "toluic acid" and "xylene" are intended to include all the various isomers and the term "lower alkyl" is defined as containing 1 to 4 carbon atoms. The oxidation may be carried out at 120-230 DEG C. and at elevated pressure in the presence of an oxidation catalyst e.g. cobalt toluate using air as the oxidising gas. In a preferred embodiment illustrated in Fig. 2 alkyl toluate is introduced into oxidiser No. 1 through A, residue from previous oxidations e.g. partially oxidised xylenes and toluates, is introduced through B, catalyst is introduced through C and the off gases from oxidiser No. 2 containing both oxygen and xylene are introduced through E. The oxidate is withdrawn from oxidiser No. 1 and passed to oxidiser No. 2 through D and additional xylene simultaneously introduced through F. The oxidate is withdrawn through G, unreacted xylene removed in the xylene stripper and the stripped oxidate esterified and then distilled to give an alkyl toluate which is recycled, a dialkyl phthalate and a high boiling residue. In examples p-xylene and methyl p-toluate are oxidised and the stripped oxidate esterified with methanol. Specification 809,730 is referred to.